Giardia lamblia is an amitochondriate, binucleate protist that causes diarrhea in the U.S. and the developing world. Our preliminary results make numerous important contributions to the understanding of the cell biology, genetics, and biochemistry of G. iamblia. Cell biology: Nuclear membranes remain intact during cell division. Giardia divides with mirror image symmetry in the plane of the adherence disc. Pairs of nuclei of dividing giardia are tethered together by arrays of microtubules. The Giardia [2Fe-2S] ferredoxin has an organelle-targeting sequence. Genetics: The GS strain, which was used to prove Koch's postulates, has symmetric nuclei that are each diploid. The WB strain, which is presently being sequenced, has asymmetric nuclei that are a mixture of monosomic and trisomic chromosomes. Four copies of each Giardia gene all have the same sequence. Giardia lack most of the machinery for sex. Giardia have a spliceosomal intron. Biochemistry: Genes encoding the majority of Giardia fermentation enzymes were laterally transferred from anaerobic prokaryotes. Giardia contain genes encoding nitroreductases, which are similar to those that activate metronidazole in bacteria. Giardia have an Fe-hydrogenase. These results suggest multiple hypotheses that will be tested in three Specific Aims. Major hypothesis for Aim 1 (cell biology): Perinuclear microtubules tether together pairs of nuclei of dividing Giardia, so nuclei are correctly distributed to mother and daughter Giardia. Major hypotheses for Aim 2 (genetics): Giardia are tetraploid but sequence like haploids; symmetric diplomonad nuclei are diploid for each chromosome (like GS); while asymmetric diplomonad nuclei have a mixture of monosomic and trisomic chromosomes (like WB). Minor hypothesis: Giardia are capable of splicing heterologous introns. Major hypothesis for Aim 3 (biochemistry): Giardia is sensitive to metronidazole and furazolidone, because of the presence of nitroreductases, like those of anaerobic bacteria, which reduce and activate these drugs.